Method for producing helical springs or stabilizers

ABSTRACT

The invention relates to a method for producing helical springs or stabilisers consisting of steel. According to said method, the parent material is heated to a temperature in excess of the re-crystallisation temperature, the structure is austenitised, held at an equalised temperature and then formed and subsequently quenched to form martensite and tempered. Round steel bars, whose re-crystallisation temperature is adjusted over the bar length in a compensation furnace, constitute the parent material. The round steel bars are subsequently re-modelled by cross-rolling, remaining substantially straight and after the critical deformation degree has been exceeded are subjected to dynamic re-crystallisation processes. The round steel bars are then subjected to a post-heating process above the Ac3 temperature, in order to undergo a complete static re-crystallisation, are wound to form a helical spring or bent to form a stabiliser and are finally quenched from the austenitic state to form martensite and tempered.

The invention relates to a method of the introductory portion of claim 1 for producing coil springs or stabilizers from the steel.

A method for continuously hardening and tempering steel wire is disclosed in DE 43 40 568 C2 and contains the following steps:

-   -   rapidly heating the wire to a temperature in the austenite range         at a rate of between 85° and 100° C./second;     -   holding the steel wire in the austenite range for a period of 10         to 60 seconds;     -   quenching the steel wire to room temperature at a rate of more         than 80° C./second;     -   rapidly heating to the to the tempering temperature at a rate of         85° to 95° /second;     -   holding at the tempering temperature for a period of 60 to 100         seconds;     -   cooling the wire at the rate of more than 50° C./second, which         is customary for water cooling.

Between steps 2 and 3, the wire can be rolled closely above the Ac3 temperature. At the same time, the wire is ovalized in a first roll pass, rolled round in the second roll pass and subsequently drawn through a calibrating die.

In the DE 195 46 204 C1, a method is described for producing objects of high-strength from a quenched and tempered steel and for using this method for producing springs. The steel with (in weight percent) 0.4 to 0.6% carbon, up to 1% silicon, up to 1.8% manganese, 0.8 to 1.5% chromium, 0.032 0.10% niobium and 0 to 0.2% vanadium, the remainder being iron, is produced in the following manner:

-   -   the raw material is solution annealed in the austenite region at         temperatures from 1050° to 1200° C.;     -   immediately afterward, the raw material is deformed hot in a         first step at a temperature above the recrystallization         temperature;     -   immediately afterward, the raw material is deformed hot in a         second step at a temperature below the recrystallization         temperature but above the Ac3 temperature;     -   the rolled material subsequently is maintained at a temperature         above the Ac3 temperature for carrying out further transforming         and processing processes and then     -   cooled to below the martensite temperature,     -   whereupon it is tempered.

The DE 196 37 968 C2 discloses a method for the high temperature, thermomechanical production of spring leaves for leaf springs and/or leaf spring linkages. The method is based on a two-step thermomechanical, production of parabolic springs and comprises the following steps:

-   -   the starting material is heated at a rate of between 4°         C./second and 30° C./second to the austenitizing temperature;     -   the austenitizing temperature is 1100°±100° C.;     -   the material is cooled from the austenitizing temperature to the         temperature of the first rolling step at a cooling rate between         10° C./s and 30° C./s.     -   Then, in the first rolling step, at a temperature of 1050°±100°         C., with a non-constant changing of shape between 15% and 80%,         it is roughed down in one or more passes.     -   It is cooled from the temperature of the first rolling step to         the temperature of the second rolling step at a rate of between         10° C./second and 30° C./second.     -   In the second rolling step, it is finish-rolled with rolls         adjustable under load, at a temperature of 880°±30° C. with a         change of shape, constant over the length of the spring leaf, of         between 15% and 45%.

Finally, the DE 198 39 383 C2 discloses a method for the thermomechanical treatment of steel for torsionally stressed spring elements, wherein the starting material is worked at a temperature above the recrystallization temperature and then reshaped at such a temperature above the recrystallization temperature in at least two transformation steps, that a dynamic and/or static recrystallization of the austenite results. The austenite of the converted product, so recrystallized, is quenched and annealed. A silicon-chromium steel is to be used, which has a carbon content of 0.35 to 0.75% and is inicroalloyed with vanadium or other alloying element.

The methods to be taken from the state of the art for the thermomechanical treatment of objects consisting of steel are based essentially on multiple converting steps, repeated cooling and heating of the starting material being necessary in order to produce the parameters obtained later on in the end product.

It is an object of the invention to make a method available for the production of coil springs or stabilizers of steel of the introductory portion of claim 1, the method permitting a targeted improvement in the property parameters directed to the loading profile of the end product.

This objective is accomplished by a method with the distinguishing features of claim 1.

Advantageous developments and embodiments of the method are described in claims 2 to 21.

For the inventive method, the starting material is first heated to a temperature above the recrystallization temperature and subsequently the temperature is equalized over the entire length of the rod. Furthermore, the temperature, to which the rod is heated, is kept constant virtually up to the entry of the rod into the roll gap. With these working steps a highly uniform structure of the rod is sought, both over its length and over its cross section, before it enters the roll gap. This is of advantage for the transformation process that follows. On account of the process-specific peculiarities of the skew rolling and due to a targeted establishment of the rolling parameters, a predetermined twisting of the material in the marginal area of the rods and a transformation gradient over the cross section of the rod set in during the one-step transformation process. Since the direction of transformation during the skew rolling is at an angle to the axis of the material rolled and the maximum of the transformation is in the marginal region of the rods, the structural stretching in this marginal zone, caused by the transformation, is especially greatly pronounced and the structural alignment corresponds to the transformation direction and also extends at an angle to the axis of the rolled material. After the critical degree of transformation is exceeded, the dynamic recrystallization process takes place with special intensity in this marginal zone, so that a gradient of the degree of recrystallization from the outside to the inside may be noted over the cross section of the rod. In the reheating following the transformation process to a temperature above Ac3 , the static recrystallization is completed and leads to the formation of fine-grained austenite, especially in the marginal zone. After hardening followed by tempering, the marginal zone is characterized by a fine martensite structure of great strength.

The invention has considerable advantages over the solutions known from the state of the art. As a result of the combination of a targeted, one-step transformation by means of skew rolling and a heat treatment coordinated therewith, the treated rods have a strength profile over their cross section, which reaches its maximum values in the marginal area. The direction of the twist of the structure produced by the skew rolling in the marginal region of the round rods corresponds to the main direction of stress of a component subjected to torsion, and the property features developed by the rods as a result thus provide optimum prerequisites for their use especially in the spring industry. The distribution of structures over the cross section of the rod produced by the inventive method results in a property profile in the completely processed round rods, which is adequate for the stress profile over the cross section of the rod during bending and torsional stresses. Stabilizers or coil springs, produced from such a steel, may have a lesser weight for the same load.

Since only a transformation step is necessary for the development of these advantageous strength effects, and the working steps that follow are performed essentially at an elevated temrperature, only a heating process for the starting material is therefore necessary. This leads to considerable savings of energy and time resulting from the procedure itself. The inventive method is distinguished therefore from known methods not only by an improvement in the stress-oriented strength and toughness properties of the finished product, but also by economic advantages offered by the minimal number of process steps.

Advantageously, the starting material, in the form of round rods, is heated inductively at a rate of 100° to 400° K/s to a temperature between 700° and 1100° C. Subsequently, the heating temperature of the rod is equalized over its length during a period of at least 10 seconds. With that, it is ensured that the temperature difference does not exceed 5° K over the length of the rod. By means of suitable reheating equipment, the heating temperature of the rod is kept constant until it enters the roll gap. The transformation itself is performed by skew rolling in a single step, in which the rods run through the roll gap, remaining straight. Depending on the quality of the starting material, the transformation is carried out preferably at a temperature ranging from 700° to 1150° C. The ratio of the starting diameter to the finished diameter is selected so that the skew rolling of the rods is performed with a mean degree of stretching λ of more than 1.3, and so that the maximum transformation amounts to ψ=0.3. By the targeted setting of the rolling parameters, such as the rotational speed of the rolls and the rate of feed, and by the special selection of roll contours with specific angular relationships, it is brought about that the maximum transformation in the marginal region is between 0.65 and 1.0 of the diameter of the rods, and that a desired transformation gradient is established over the cross section of the rod. Preferably, the skew rolling process is controlled so that a maximum local temperature increase of 50° K is not exceeded in the rolled material.

Due to the transformation, after a critical degree of transformation degree is exceeded, dynamic recrystallization processes take place, which, on account of the maximum transformation, are more strongly pronounced in the marginal zone than in the core region of the rods. The targeted influencing of the formation of a transformation gradient over the cross section of the rod has the result that the first indications of a differentiated structure distribution appear across the cross section of the rod already in the course of the dynamic recrystallization. Thus, metallographic studies of rods in the recrystallized state, which have been rolled pursuant to the invention, show that the proportion of fine austenite crystals decreases clearly from the marginal zone toward the core region.

The differentiated structural formation across the cross section of the rolled material is furthermore additionally intensified by a typical peculiarity of skew rolling. Since the direction of transformation runs at an angle to the axis of the rolled material in skew rolling, a striking stretching of structure occurs especially in the marginal areas of the material rolled due to the greater degree of transformation corresponding to the direction of transformation. The structure is also stretched at an angle to the axis of the rolled material and leads to a twisting of the material in the marginal zones. In the course of the inventive process, the direction of the twisting of the structure in the marginal region of the rods is 35 to 65 degrees of angle with respect to the longitudinal axis of the rod and thus corresponds to the main direction of stress of a component subjected to torsion.

In the process of single-step skew rolling, the entire length of the rod being rolled runs through a roll gap of constant geometry. This procedure is selected whenever rods with uniform diameter over their entire length are to be produced. The inventive method furthermore makes an alternative variation of the process possible, in which the roll gap geometry is varied in the operating state while the rod to be rolled is passing though the roll gap. This flexible manner of operation is achieved with a skew roll stand, the rolls of which can be adjusted in the axial and/or radial direction as needed during the transformation. The inventive method thus permits round rods to be produced, the diameter of which varies over the length of the rods.

Immediately after they exit from the roll stand, the skew-rolled rods are subjected to reheating at a temperature above Ac3 in such a manner, that the temperature difference over the length of a rod is limited to 5° K.

Depending on their later intended use, the rods, skew-rolled and reheated to the recrystallization temperature, are either coiled hot to form coil springs or bent to form a stabilizer.

The coiled or bent components are then hardened and afterward tempered.

Rods, which are intended for manufacturing torsion bar springs, are mechanically worked at their ends in the cold state after reheating, then heated to above Ac3 , quenched and tempered.

Macro-examinations of the finished rods show a typical distribution of structures over the cross sections of the rods as a consequence of the inventive combination of skew rolling and heat treatment. The immediate marginal zone has fine-grained martensite structure of high strength. The marginal area has a continuous stretching of structure extending at an angle to the axis of the rod, the direction of twist corresponding to the main direction of tension of a torsionally stressed component The mixed pearlite-martensite structures of the core zone are coarser than the structures in the marginal area and exhibit no twisting phenomena.

To provide optimum toughness and strength parameters in the finished product, round rods of spring steel, preferably silicon-chromium steels with carbon contents of less than 0.8%, are used as starting material in the inventive method. Alternatively, these steels can be micro-alloyed with vanadium or niobium

The inventive object is represented by an embodiment in the drawing and is described as follows.

The sole FIGURE shows the diagrammatic arrangement of a continuous working line for the inventive thermomechanical treatment of round steel rods of a silicon-chromium steel.

The rods to be treated are heated in an induction apparatus 1 to a temperature above the recrystallization temperature, while their structure is austenitized. In the present example, the round steel rods are heated at a rate of 130° K/s to a temperature of 980° C. In an equalization furnace 2 following the induction apparatus 1, the heating temperature of the rods is equalized over a period of 15 s, so that the course of the temperature over the length of the rods has a gradient of 4° K.

In this state the round steel rods, now uniform by tempered, are brought into a holding oven 3 to keep their temperature constant until they enter the roll gap. The heated rods are transported by means of gang rolls 6 and 7, both in the equalizing oven 2 and in the holding oven 3.

In a skew rolling stand 4, the round steel rods, heated to 980° C., are shaped in a rolling step. At the same time the ratio of the starting diameter to the finished diameter is chosen so that the average degree of stretching X is 1.5 and that the maximum transformation ψ is at least 0.3. By the targeted setting of rolling parameters, such as the roller speed or the rate of feed and by the special selection of roller contours with specific angular relationships, the maximum transformation in the marginal region between 0.65 and 1.0 of the diameter of the rods is achieved and thus a desired transformation gradient is established over the cross section of the rod. The rolling parameters are coordinated with one another so that a maximum local temperature increase of 50° K due to the transformation process is not exceeded in the material rolled. The direction of transformation at an angle to the rolling axis during the skew rolling produces in the marginal regions of the material rolled a pronounced stretching of its structure because of the greater transformation. Corresponding to the direction of transformation, this stretching of structure likewise runs at an angle to the axis of the rolled material and, in the marginal regions of the rods, results in a twisting of the material. In the course of the inventive process, the direction of the twisting of the structure, with respect to the longitudinal axis of the rods, amounts to 35 to 65 degrees of angle and thus corresponds to the main direction of stress of a component subjected to torsion.

After they exit from the skew rolling stand 4, the rolled rods pass into a reheating furnace 5 downstream from the stand, in which they are reheated above the Ac3 temperature to assure complete static recrystallization. The rods are transported through the reheating furnace 5 by means of a roller conveyor 8. After leaving the reheating furnace 5 the skew-rolled rods are transported further on a transfer roller conveyor 9. From this transfer roller conveyor 9, the rods are delivered to the rest of the intended processing steps.

FIG. 1 diagrammatically shows a production line for producing wound coil springs. Accordingly, the rods are passed over the transfer roller conveyor 9 to a lift table 10 and pass from there into a CNC winding bench 11, where the hot winding to coil springs takes place after the recrystallization. After the winding process, the rods, now wound into coil springs, are transferred to a hardening vat 12, in which they are quenched and their structure is converted to martensite. The hardened coil springs are then subjected to a tempering treatment, which is not shown.

List of Reference Numbers

-   1. Induction apparatus -   2. Equalization furnace -   3. Holding oven -   4. Skew rolling stand -   5. Reheating furnace -   6. Gang rolls -   7. Gang rolls -   8. Gang rolls -   9. Transfer roller conveyors -   10. Lift table -   11. CNC winding bench -   12. Hardening vat 

1. Method for producing coil springs or stabilizers of steel, wherein the starting material is heated to a temperature; above the recrystallization temperature, austenitized, held for equalization of temperature, then deformed and finally quenched to martensite and tempered, characterized by starting out with round steel rods, the heating temperature of which is equalized over the rod length and which then are transformed by skew rolling, while remaining approximately straight, so that a predetermined twisting of the material in the marginal area and a desired transformation gradient is achieved over the cross section, and, after the (critical) degree of transformation is exceeded, dynamic recrystallization processes take place, whereupon the rods are reheated to a temperature above Ac3 , wound into a coil spring or bent into a stabilizer in order finally to be hardened and tempered.
 2. Method of claim 1, characterized in that the direction of the twisting of the structure in the marginal region of the round rod corresponds to the main direction of tension of the coil spring or the stabilizer stressed by torsion.
 3. Method of claim 1, characterized in that the direction of twist of the structure in the marginal region, with respect to the axis of the round rod, amounts to 35°-65°.
 4. Method of 1, characterized in that the skew rolling is carried out in one step.
 5. Method of claims 1, characterized in that the skew rolling of the rod is performed with an average degree of degree of stretching λ of at least 1.3.
 6. Method of claims 1, characterized in that the maximum transformation in the marginal area amounts to between 0.65 and 1.0 times the diameter of the rod and is at least 0.3.
 7. Method of claims 1, characterized in that the material is heated at a rate between 100°-400° K/s.
 8. Method of claims 1, characterized in that the starting material is heated to a temperature between 700° and 1100° C.
 9. Method of claims 1, characterized in that the heating is performed inductively.
 10. Method of claims 1, characterized in that the equalization of the heating temperature of the rod takes place for at least 10 seconds.
 11. Method of claims 1, characterized in that the temperature difference over the length of the rod does not exceed 5° K.
 12. Method of claims 1, characterized in that the heating temperature of the rod is kept constant virtually up to its entry between the rolls.
 13. Method of claims 1, characterized in that, during the skew rolling, a maximum local temperature increase of 50° K is not exceeded.
 14. Method of claims 1, characterized in that the skew rolling is performed in a temperature range of 700°-1000° C.
 15. Method of claims 1, characterized in that the rolls of the skew rolling stand are adjusted in the axial and/or radial direction during the transformation operation and the round rods are produced with a diameter, which varies over their length.
 16. Method of clams 1, characterizd in that, during a reheating above Ac3 following the skew rolling, the temperature difference over the rod length is limited to a maximum of 5° K.
 17. Method of claims 1, characterized in that it starts out from spring steel.
 18. Method of claims 1, characterized in that it starts out from a silicon-chromium steel.
 19. Method of claims 1, characterized in that it starts out from a microalloyed steel.
 20. Coil spring, produced by a method of claims 1, characterized in that, under load, it has almost the same stress distribution over the cross-section.
 21. Stabilizer, produced by a method claims 1, characterized in that, under load, the part, stressed in torsion, it has almost the same stress distribution over the cross-section. 